You can low stress train in week two or three, quite early on. The seedling is delicate when it’s just new... so gentle at first is a good bet. For example, when the plant is just a few inches tall you can tie the top down so that it just leans over to the side slightly. The light stress on the stem with make it fatter and the extra light will allow the lower side branches to stretch. She will correct herself pretty fast, several hours even, turning her leaves back towards the light.